Fhv. statsgeolog og fhv, direktør for Forskningsstyrelsen: Tom Snak om Ny Syndflod

Hvis jeg skal skære den ejendommelige debat i Jyllands-Posten ud i pap, så handler den om, at jeg i min kronik (17/10) bl.a. bebrejdede IPCC, at man fravælger de mere end 2.000 århundredlange måleserier af vandstandsdata fra havne over hele kloden og erstatter de seneste 25 år af disse simple og derfor troværdige målinger med langt mere usikre satellitmålinger af oceanernes vandstand.

Det korte af det lange er, at havnemålingerne viser, 1) at havene langs kysterne kun stiger 1,0-1,5 mm om året, 2) at havstigningen gennemsnitligt har svinget i dette interval, siden havnemålingerne begyndte for mere end 200 år siden, og 3) at der ikke er nogen målbar acceleration i nutiden.

Abstract
A comprehensive set of 456 monthly tide gauge records is analyzed for trend and acceleration over the same period that satellite altimetry was analyzed (1993 to 2011). Additionally, a 90 tide gauge record subset is analyzed for which GPS data are available. The selection criterion for the tide gauge data is 85% data completion. All measurements are adjusted for vertical land motion. Results from 456 pairs of tide gauges, adjusted for Global Isostatic Adjustment, and satellite recordings located within 1° root-mean-square latitude and longitude separation differences are compared. The tide gauge trends and accelerations are adjusted for spatial bias using the more globally dense satellite data.

The average trends of the 456 and 90 gauge sets (3.26 and 2.68 mm/year, respectively) agree reasonably well with the global trend average of the satellite data (3.09 mm/year). Average trends for the 456 tide gauges are also in good agreement (within 95% confidence limits) with trends based on satellite data within the 1° satellite proximity criterion (3.26 and 3.31 mm/year, respectively). The trends for the 90 gauges with GPS nearby and qualifying satellite locations are 2.68 and 2.74 mm/year, respectively. For all datasets analyzed, the accelerations are quite strongly negative but the uncertainty is relatively large. Adjustment of the tide gauge trends for spatial bias modified both trends and accelerations significantly and decreased trend differences between the 456 and 90 gauge datasets. The spatially adjusted tide gauge trends (2.95 and 2.72 mm/year, respectively for the 456 and 90 tide gauges sets) are somewhat less than the 1° spatially adjusted satellite data (3.09 mm/year). Whether the increased sea level trend of approximately 3 mm/year measured by the satellites since the 1990's is a long-term increase from the 20th Century value of approximately 1.7 mm/year or part of a cycle will require longer records; however, the negative accelerations support some cyclic character.

A new study led by the University of Siegen (Germany) finds an acceleration in sea-level rise starting in the 1960s that can be linked to changes in Southern Hemispheric westerly winds. The study, published on August 5 in the journal Nature Climate Change, examined a global set of coastal tide gauge records in combination with spatial information from satellite altimetry to retrace the acceleration in time and space.

"Since the early 1990s, satellites have measured sea level with high precision and good spatial coverage over almost the entire ocean, and scientists have found that its global average has been accelerating over the last 25 years, mainly as a result of increased melting in Greenland and Antarctica," says Sönke Dangendorf, a scientist at the University of Siegen and lead author of the paper. "But, so far, it has been unclear when this acceleration started, in which region it was initiated, and which processes contributed the most to it. Answering these questions was hampered by the fact that before the advent of satellite altimetry in 1992, our knowledge mainly relied on a few hundred tide gauges that record sea level along the world's coastlines, and that our available approaches for reconstructing sea levels globally from these records were too imprecise."

kfl skrev:
Det er korrekt , at der ligger et paradoks i at vandstandsstigninger langs kysterne er mindre end i oceanernes midte. Det må nogen kunne redegør for.

Abstract
We use 1277 tide gauge records since 1807 to provide an improved global sea level reconstruction and analyse the evolution of sea level trend and acceleration. In particular we use new data from the polar regions and remote islands to improve data coverage and extend the reconstruction to 2009. There is a good agreement between the rate of sea level rise (3.2 ± 0.4 mm·yr− 1) calculated from satellite altimetry and the rate of 3.1 ± 0.6 mm·yr− 1 from tide gauge based reconstruction for the overlapping time period (1993–2009). The new reconstruction suggests a linear trend of 1.9 ± 0.3 mm·yr− 1 during the 20th century, with 1.8 ± 0.5 mm·yr− 1 since 1970. Regional linear trends for 14 ocean basins since 1970 show the fastest sea level rise for the Antarctica (4.1 ± 0.8 mm·yr− 1) and Arctic (3.6 ± 0.3 mm·yr− 1). Choice of GIA correction is critical in the trends for the local and regional sea levels, introducing up to 8 mm·yr− 1 uncertainties for individual tide gauge records, up to 2 mm·yr− 1 for regional curves and up to 0.3–0.6 mm·yr− 1 in global sea level reconstruction. We calculate an acceleration of 0.02 ± 0.01 mm·yr− 2 in global sea level (1807–2009). In comparison the steric component of sea level shows an acceleration of 0.006 mm·yr− 2 and mass loss of glaciers accelerates at 0.003 mm·yr− 2 over 200 year long time series.

Det korte af det lange er, at havnemålingerne viser, 1) at havene langs kysterne kun stiger 1,0-1,5 mm om året, 2) at havstigningen gennemsnitligt har svinget i dette interval, siden havnemålingerne begyndte for mere end 200 år siden, og 3) at der ikke er nogen målbar acceleration i nutiden.

Det store spørgsmål er fortsat: Hvordan kan satellitter, som svæver mange hundrede eller mange tusinde kilometer over jorden, måle vandstanden i havene mere præcist en de landbaserede målinger kan?